CATV systems typically include a "headend" for acquiring and preparing television signals for routing to customers or other viewers and a distribution system for carrying the prepared signals to the customers' receivers. The television signals are sent in a frequency-domain multiplexed format over a coaxial cable, i.e., each signal is converted to a unique frequency for transmission. To receive a given signal on the cable, the customer tunes the receiver to the desired signal channel in the same manner as for a broadcast signal.
Tuning to a channel entails receiving a number of signals confined within the channel while excluding signals confined within adjacent channels. FIG. 1 shows a typical distribution of signals confined within representative adjacent channels of the television frequency spectrum. Each of the channels, in this case channels two, three, and four, includes a picture carrier P, a chroma carrier C, and a sound carrier S. Each carrier may further include sideband SB energy resulting from modulation of the carrier. An alphanumeric prefix to a sideband SB identifier associates the sideband with a particular carrier, and a numeric subscript associates the carrier or sideband with a channel number. For example, PSB.sub.3 is the numeric designation for the channel 3 picture carrier sidebands. Carriers C.sub.2 and S.sub.2 are unmodulated and therefore have no sidebands. Carrier P.sub.2 is modulated by sync pulses only and, therefore, shows sidebands PSB.sub.2 having a cusped distribution characteristic of pulsed modulation. Carrier P.sub.3 is modulated by sync pulses and video, and therefore, shows sidebands PSB.sub.3 having an irregular distribution characteristic of video modulation.
At any instant in time, the signal energy distribution in a particular channel may appear like that shown for channel two, three, or four depending on the modulation present at a particular instant in time. The signal energy distribution shown for channel two represents the minimal modulation present during a vertical retrace interval, whereas the signal energy distribution shown for channel three represents the typical modulation present during an active horizontal line. The signal energy distribution shown for channel four represents the modulation present during a vertical retrace interval in which a multiburst color test signal is present. Therefore, sidebands PSB.sub.4 have a cusped distribution characteristic of such a test signal. Of course, many other energy distribution patterns are possible.
The distribution system requires that the CATV signals, such as those illustrated in FIG. 1, be amplified at regular distances along the cable to restore signal strength lost because of cable attenuation. CATV amplifiers typically have low distortion and noise, but even high-quality amplifiers add distortion and noise to the amplified signal.
Distortion generates additional signals referred to in the CATV industry as intermodulation products or "beats." Among the most troublesome are "triple beats" caused by mixing together three different signals. Picture carriers P are the strongest signals carried on the cable. Because most of the picture carrier frequencies are spaced apart by six megahertz, most triple beat products fall very close to the picture carrier frequencies. In a CATV band spanning 500 MHz, over 1000 beats can occur near each picture carrier frequency. With so many beats, no attempt is made to measure them individually, but rather the composite total of all the beats is measured.
To increase signal-to-noise ratio, the signal amplitude on the cable should be maximized. However, large signal amplitudes cause the triple beat amplitudes to increase sharply, causing a "muddy" television picture. To minimize the effects of triple beats, the amplitude of the signals should, therefore, be minimized. However, minimizing the amplitude of the signals drops their amplitude with respect to the noise causing a "snowy" television picture.
Therefore, optimum performance in a CATV distribution system requires carefully adjusting the signal amplitude to balance the effects of distortion and noise.
Prior methods for measuring noise and/or distortion required interrupting service on the CATV channel being measured. Because customers complained about such loss of service, "in-service" techniques were developed so that measurements could be made without interrupting service.
For example, and with reference to FIG. 2, U.S. Pat. No. 5,073,822 issued Dec. 17, 1991 for IN-SERVICE CABLE TELEVISION MEASUREMENTS, which is assigned to the assignee of this application, describes a spectrum analyzer 10 equipped to make "gated spectrum measurements" that allow energy to be sampled from a channel at predetermined time intervals such as during a vertical interval or an active line time. Gated spectrum measurements are particularly useful for making measurements during time intervals when no modulation is present.
Spectrum analyzer 10 receives an RF input signal that is attenuated, filtered, and mixed with a swept local oscillator 12 that up-converts the frequency of the input signal to a first intermediate frequency. The first intermediate frequency is then down-converted by a second fixed frequency local oscillator 14 to produce a second intermediate frequency. The second intermediate frequency is mixed with a system clock signal CLK to produce a final intermediate frequency signal for processing and display.
The final intermediate frequency signal is conditioned by a variable bandwidth IF amplifier 16 and a logarithmic amplifier 18 for distribution to a video amplifier 20 and a trigger circuit 22. The output signal from video amplifier 20 is either stored in a digital storage device 24 or sent to a deflection amplifier 26 for display on a cathode-ray tube (CRT) 28 or other suitable display device. The output signal from trigger circuit 22 is sent to a sweep circuit 32 for triggering sweep signals that coordinate the first local oscillator tuning frequency with the video signal display. The displayed signal originates either directly from video amplifier 20 or indirectly from digital storage device 24.
Making noise measurements entails tuning spectrum analyzer 10 to an unswept frequency above the picture carrier frequency. However, such tuning prevents a sync separator circuit in trigger circuit 22 from operating normally. Therefore, a separate trigger extraction circuit 33 is added to the otherwise conventional circuit. Trigger extraction circuit 33 includes an IF amplifier 36 coupled to the output of a mixer 38, a video detector 40 coupled to the output of IF amplifier 36, and a video amplifier 42 coupled to the output of video detector 40. The output signal polarity of video amplifier 42 is selected by a polarity switch 44 and is coupled to deflection amplifier 26 and via a trigger selector switch 46 to a sync separator (not shown) in trigger circuit 22. This arrangement provides selectable time interval triggering of the display sweep signals during, for example, the unmodulated vertical interval time of the channel being measured.
A transmission gate 48 situated in the IF signal path passes signal energy only during the predetermined measurement time periods, such as the vertical interval or an active line time. Transmission gate 48 is controlled by a gate control circuit 49 that is coupled to the output of trigger circuit 22. With this circuit arrangement, spectrum analyzer 10 can provide in-service cable television measurements of carrier-to-noise ratio and composite triple beat ratio. The carrier-to-noise ratio is determined by averaging signal data sampled at a fixed time interval of each unmodulated horizontal line time, and the triple beat ratio is determined from signal data sampled during the unmodulated vertical interval.
In spectrum analyzer 10, IF amplifier 36 is fixed-tuned to the frequency of the picture carrier to provide triggers that allow measuring spectral energy centered in the channel being measured. However, other types of measurements, such as spectral analysis of the multiburst color test signal represented in channel three of FIG. 1, require different frequency offsets from the carrier frequency. Spectrum analyzer 10 cannot make such measurements because other signals, such as adjacent channel picture or sound carriers, may interfere with extracting a stable trigger signal.
What is needed, therefore, is an in-service radio-frequency signal measurement system capable of performing gated spectral measurements on signals, such as the multiburst color test signal, that are distributed at frequencies throughout the channel being measured. The system should also be useful for broadcast signal measurements such as those found in cellular radio applications.